The Scatter-based series in the XamDataChart in Infragistics for Xamarin.Forms does not currently support tooltips at the moment. This is documented at the following article, where there is a note stating that tooltips are not currently supported for Polar or Scatter series: https://www.infragistics.com/help/xamarin/datachart-chart-tooltips.

The current issue in this case is that the internal Infragistics.Controls.DataContext type that is assigned as a BindingContext to your elements that you place in your ToolTipTemplate is not currently assigned an Item when using Scatter and Polar series, and so unlike the category-based series, the Item property that you are binding to will consistently return null. If your data is of a linear structure, I would heavily recommend utilizing a LineSeries if you need to show tooltips in the Infragistics for Xamarin.Forms XamDataChart control.

I believe this will be implemented in a future version of Infragistics for Xamarin.Forms, but if you would like to make your desire for this feature heard, I would recommend suggesting a new product idea for it by e-mailing ideas@infragistics.com. This will place you in direct communication with our product management teams who plan and prioritize upcoming features and development based on community and user feedback.

Please let me know if you have any other questions or concerns on this matter.